extrude, intersect, slice complex shapes?
I think that the solution to this is Blender's inset function. The goal is to separate all faces equidistantly from another and do this in an efficient way. 

Please be aware of the fact, that Blender is not intended to be a precision CAD software. The primary use is and always will be 3D modeling and rendering. 

In the first step we need to change the unit settings to the Units we want to take our measures in (Properties Window - Scene Panel). In this example this will be "Metric", "Degrees" and a conversion scale of 1.0



We also want to apply the scale of the object to ensure that the values will be applied in the right scale. Hit Ctrl+A and select "Scale. 



This is how your object's Transform values should change.

The next step is to separate the faces from each other. Tab into Edit Mode, type A to select all faces, hit Space, type "Edge Split" and hit Enter



Now that we have separated faces in our mesh we can inset them. Blender is not capable to do this in one step but the method we are going to use is still fairly easy.

If something else is selected, switch to "Face Select", type A to select all Faces, then I to start Face inset. Use the mouse to resize the inset value to approximately the desired size. If the gap's size between the faces should be 3mm go to the operator panel and set "Thickness" to 1.5mm (type in both the value and the unit here). Do not select or deselect any faces after that!



The last step is to delete the newly created redundant faces. Type Ctrl+I to invert the selection, hit X to delete and choose "Faces". 



As a result you should get a model which is made out of separated and evenly spaced faces. 



The described method works also on ngons. 



To get a cutting pattern for your Tiffany work, you could uv-unwrap your model, increase the margin width between the islands and export it as svg. With inkscape you would be able to resize the pattern and move the pieces around until you get something useful.